IN the course of our work on the chemistry of the constituents of the cell nucleus, nucleoprotein has been extracted from cell nuclei of normal and tumour tissue and fractionated into three main components: nucleic acid, the basic ‘histone’ protein and an acid-insoluble protein residue1. Quantitative amino-acid analyses are being carried out on these fractions. It was thought that the results obtained for calf thymus histone would be of value, since the results hitherto available account for only 50 per cent of the total composition2.